The present invention relates to electrocoating compositions containing a resinous phase dispersed in an aqueous medium, the resinous phase comprising an ionic electrodepositable resin and a novel polysiloxane component, and to their use in the method of electrodeposition. More particularly, this invention relates to electrocoating compositions which have improved crater resistance.
Electrodeposition as a coating application method involves deposition of a film-forming composition onto a conductive substrate under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers increased paint utilization, improved corrosion protection and low environmental contamination.
Initially, electrodeposition was conducted with the workpiece being coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has steadily gained in popularity and today is by far the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.
Although surface coatings of excellent quality can be achieved by means of cationic electrodeposition, a problem associated with this means of coating is the development of surface defects upon curing, particularly craters. The cause of such surface defects can be a result of the very nature of the components of the electrocoating composition, that is, causes inherent in the system. Typically, however, the cause of such surface defects is the impurities which are carried into the electrocoating bath with the workpiece. Examples of such impurities can include lubricating oil, anti-corrosion grease, joint sealing compounds and the like.
As the electrocoating composition is deposited onto the conductive substrate, the impurities are carried along with the coating composition and are deposited as well. When the coated substrate is cured, craters are formed due to incompatibility between the impurity and the resinous phase of the electrocoating composition.
The use of silicone additives such as SILWET L-7602, a silicone oil commercially available from OSi Specialties, a subsidiary of Whitco Corporation, to prevent crater formation is well known in the art. Although these silicone additives are quite effective for crater control in electrocoating compositions, they also pose the serious disadvantage of intercoat adhesion failure of subsequently applied coating layers.
Japanese Patent Application J61,115,974 discloses the addition of a reaction product formed from a polyepoxide resin modified with dimeric fatty acids and a polyoxyalkylene polyamine to cationic electrocoating compositions to suppress crater formation. These products, however, can result in intercoat adhesion failure of subsequently applied primers and/or topcoats. U.S. Pat. No. 4,432,850 discloses the addition of a reaction product of a polyepoxide resin with a polyoxylene-polyamine containing primary amino groups to cationic electrocoating compositions to eliminate or minimize crater formation. However, to achieve maximum effectiveness as a crater control additive, this polyepoxide-polyoxyalkylenepolyamine resin sometimes must be added at quite high levels (i.e., 0.5 to 40 percent by weight), thereby diluting the properties of the other film-forming cationic resins which are present in the electrodepositable coating composition.
U.S. Pat. Nos. 5,427,661 and 5,501,779, both to Geist et al., disclose the addition of a homopolymer or copolymer of an alkyl vinyl ether to cationic electrocoating compositions to suppress the occurrence of craters in the deposited surface coating materials. Such alkyl vinyl ether materials provide adequate crater control while overcoming the intercoat adhesion failure observed with the above-mentioned crater control additives. However, these alkyl vinyl ether materials can impart the tendency in a coating to telegraph solvent wipe marks and negatively affect wettability of a subsequently applied primer or topcoat.
Polysiloxane polyols are well known in the art. Japanese Patent Publication 48-19941 describes polysiloxane polyols which are obtained by the dehydrogenation reaction between a polysiloxane hydride and an aliphatic polyhydric alcohol or polyoxyalkylene alcohol to introduce the alcoholic hydroxy groups onto the polysiloxane backbone. These polysiloxane polyols are disclosed as being useful as non-ionic surface active agents.
U.S. Pat. No. 4,431,789 to Okazaki et al. discloses a polysiloxane polyol which is obtained by the hydrosilylation reaction between a polysiloxane containing silicon hydride and a polyglycerol compound having an aliphatically unsaturated linkage in the molecule. Examples of such polyglycerol compounds are those obtained by the reaction of allyl alcohol and glycidol or by the reaction of diglycerin and allyl glycidyl ether. This reaction, a so-called hydrosilylation reaction, is an addition reaction between an organosilicon compound having a hydrogen atom directly bonded to the silicon atom, i.e., a polysiloxane hydride, and an organic compound having aliphatic unsaturation in the molecule carried out in the presence of a catalytic amount of a Group VIII noble metal. The resulting polysiloxane polyols are also useful as non-ionic surface active agents.
U.S. Pat. No. 5,260,469 discloses butoxylated polysiloxane polyols which are disclosed as being useful in cosmetics. U.S. Pat. No. 5,248,789 discloses epoxy functional polysiloxanes which are formed by reacting a polysiloxane-containing silicon hydride with allyl glycidyl ether. Due to their high refractive indices, the resulting epoxy-functional polysiloxanes are useful as components in high gloss coatings and optical coupler adhesives.
U.S. patent application Ser. No. 08/904598 discloses organic polysiloxane polyols obtained by the hydrosilylation reaction of a silicon hydride containing polysiloxane with an alkenyl polyoxylene alcohol. Such polysiloxane polyols are useful in coating compositions where they provide improved mar resistance and cure response. U.S. patent application Ser. No. 08/904596 discloses acetoacetate functional polysiloxanes obtained by the transesterification of the polysiloxane polyols described immediately above with an acetoacetate. These acetoacetate functional polysiloxanes are useful in ambient-cured coating compositions which also contain a polyketimine crosslinker. U.S. patent application Ser. No. 08/904597 discloses coating compositions which comprise organic polysiloxanes which can have pendant groups containing a variety of reactive functional groups such as OH, COOH, NCO, carboxylate such as ester, carbonate and anhydride, primary amine, secondary amine, amide, carbamate and epoxy, functional groups. These functional polysiloxanes, when combined with curing agents, provide thermosetting coatings with good physical properties.
The prior art references do not teach the use of either the polysiloxane polyols or their derivatives as components in electrocoating compositions to provide improved crater control. Moreover, there is no reference which disclosed the hydosilylation of a low molecular weight polysiloxane containing silicon hydride with materials containing multiple unsaturated bonds to provide groups pendant from the polysiloxane backbone, which contain unsaturated bonds capable of undergoing hydrosilylation reaction.
It, therefore, would be advantageous to provide an electrocoating composition which suppresses or eliminates the occurrence of surface defects such as craters, without adversely affecting intercoat adhesion of primers and/or topcoats which may be subsequently applied to the deposited coating material. It would also be advantageous to provide low molecular weight polysiloxanes having pendant groups containing one or more unsaturated bonds capable of undergoing hydrosilylation reaction.